System and method to provide tight locking for DLL and PLL with large range, and dynamic tracking of PVT variations using interleaved delay lines

ABSTRACT

An interleaved delay line for use in phase locked and delay locked loops is comprised of a first portion providing a variable amount of delay substantially independently of process, temperature and voltage (PVT) variations while a second portion, in series with the first portion, provides a variable amount of delay that substantially tracks changes in process, temperature, and voltage variations. By combining, or interleaving, the two types of delay, single and dual locked loops constructed using the present invention achieve a desired jitter performance under PVT variations, dynamically track the delay variations of one coarse tap without a large number of delay taps, and provide for quick and tight locking. Methods of operating delay lines and locked loops are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is a divisional of U.S. application Ser. No.09/652,632 entitled “An Interleaved Delay Line for Phase Locked andDelay Locked Loops” filed Aug. 31, 2000 and having common ownership.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to phase locked and delay locked loopsand, more particularly, to the delay line used in such loops.

2. Description of the Background

A phase locked loop is a circuit designed to minimize the phasedifference between two signals. When the phase difference approacheszero, or is within a specified tolerance, the phase of the two signalsis said to be “locked”. A delay locked loop is similar to a phase lockedloop, but instead of producing an output signal which has the same phaseas an input or reference signal, the delay locked loop passes areference signal or input signal into a delay line, and the output ofthe delay line has some predefined phase delay with respect to thereference or input signal.

Phase locked loops (PLL's) and delay locked loops (DLL's) are widelyused circuits where it is necessary to have two signals which have aknown relationship to one another. For example, when transmittinginformation from a sending device to a receiving device, it is necessaryto have the local clock of the receiving device in sync with the clockof the sending device so that the information can be reliablytransmitted. A PLL may be used for that purpose. Both PLL's and DLL'shave been used for a long period of time, and numerous analog examplesof these circuits can be found in the literature and in many devices.

Both PLL's and DLL's may be implemented either by analog components ordigital components. In an analog loop, a delay chain is used to adjustdelay and each element in the delay chain has its delay varied by analogbias voltages supplied by a phase detector. In a digital loop, ratherthan adjust the delay of, for example, a transistor, the delay isadjusted based on the number of delay stages that are included in thedelay chain. Analog loops have continuous delay adjustments whereasdigital loops adjust delays in discreet steps. As a result, oneadvantage of an analog loop is that the jitter is very low compared tothe step jitter of a digital loop.

It is also known to implement loops in phases. For example, U. S. Pat.No. 6,445,231 entitled Digital Dual-Loop DLL Design Using Coarse andFine Loops illustrates a circuit in which the delay line is comprised ofboth a coarse loop and a fine loop. The coarse loop is designed toproduce an output signal having a phase variation from an input signalwithin a coarse delay stage while the fine loop is designed to producean output signal having a phase deviation from the input signal which issubstantially smaller than the deviation of the coarse loop. The coarseloop is designed to bring the output signal to a near phase lockcondition, or phase delayed condition, while the fine loop is designedto achieve a locked condition. Thus, a dual-loop (coarse and fine loops)all digital PLL or DLL can provide a wide lock range while at the sametime still providing a tight lock within reasonable time parameters.

There are several ways to implement the fine delay tap used in a fineloop. For example, one implementation embodies load-adjusting using avariable load capacitors. Another implementation is to provide both afast path and a slow path using slightly different sized devices. Thefirst method has little intrinsic delay and almost constant delay overprocess, voltage and temperature (PVT) variations. In contrast, thesecond method has a large intrinsic delay but provides better trackingfor delay variations. Thus, a tradeoff must be made which is driven bythe design parameters of the final device. Accordingly, a need existsfor a DLL and PLL that have a large locking range, tight lockingcharacteristics, little intrinsic delay, low power distribution and goodtracking over PVT variations.

SUMMARY OF THE PRESENT INVENTION

The present invention is directed to an interleaved delay line for usein phase locked and delay locked loops. The present invention iscomprised of a first portion providing a variable amount of delaysubstantially independently of process, temperature and voltage (PVT)variations while a second portion, in series with the first portion,provides a variable amount of delay that substantially tracks changes inprocess, temperature, and voltage variations. By combining, orinterleaving, the two types of delay, single and multiple locked loopsconstructed using the present invention achieve a desired jitterperformance under PVT variations, dynamically track the delay variationsof one coarse delay stage without a large number of fine delay taps, andprovide for quick and tight locking. Those, and other advantages andbenefits, will be apparent from the Description of the PreferredEmbodiment appearing hereinbelow. Methods of operating delay lines andlocked loops are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

For the present invention to be easily understood and readily practiced,the present invention will now be described, for purposes ofillustration and not limitation, in conjunction with the followingfigures, wherein:

FIG. 1 is a block diagram of a memory device in which a DLL having aninterleaved delay line constructed according to the teachings of thepresent invention may be used;

FIG. 2 is a block diagram of the DLL of FIG. 1 in conjunction withcertain components of the memory device;

FIGS. 3 and 4 illustrate two methods of implementing delay interpolationfor the fine loop of a delay line;

FIG. 5 is a block diagram illustrating an interleaved delay lineimplementing the methods shown in FIGS. 3 and 4;

FIG. 6 illustrates a circuit for implementing a locked loop having aninterleaved delay line;

FIG. 7 illustrates another method of implementing delay interpolationfor the fine loop of a delay line;

FIGS. 8A, 8B and 8C are simulations of the delay adjustment of theembodiments of FIGS. 3, 7 and 4, respectively;

FIG. 9 illustrates the present invention used in a phase locked loop;and

FIG. 10 is a block diagram of a computer system using the memory deviceof FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in conjunction with FIG. 1which illustrates a memory device 10. The reader will understand thatthe description of the present invention in conjunction with the memory10 of FIG. 1 is merely for the purpose of providing one example of anapplication for the present invention. The present invention is not tobe limited to the application shown if FIG. 1.

The memory device 10 includes, by way of example and not limitation, asynchronous dynamic random access memory device (SDRAM). As shown inFIG. 1, memory device 10 includes a main memory 12. Main memory 12typically includes dynamic random access memory (DRAM) devices whichinclude one or more memory banks, indicated by BANK 1-BANK N. Each ofthe memory banks BANK 1-N includes a plurality of memory cells arrangedin rows and columns. Row decode 14 and column decode 16 access the rowsand columns, respectively, in response to an address, provided onaddress bus 18 by an external controller (not shown), such as amicroprocessor. An input circuit 20 and an output circuit 22 connect toa data bus 24 for bi-directional data communication with main memory 12.A memory controller 26 controls data communication between the memory 10and external devices by responding to an input or reference clock signal(CLKref) and control signals provided on control lines 28. The controlsignals include, but are not limited to, Chip Select (CS*), Row AccessStrobe (RAS*), Column Access Strobe (CAS*), Write Enable (WE*), andClock Enable (CKE).

A digital locked loop DLL 30, constructed according to the teaching ofthe present invention, connects to input circuit 20 and output circuit22 for performing a timing adjustment, such as skew elimination or clocksynchronization between two clock signals. While the invention isdescribed in the context of a DLL, the present invention is applicableto any type of PLL. According to the teachings of the present inventionDLL 30 is an all digital loop. Those skilled in the art will readilyrecognize that the memory device 10 of FIG. 1 is simplified toillustrate the present invention and is not intended to be a detaileddescription of all of the features of a memory device.

FIG. 2 is a block diagram illustrating a portion of memory device 10 ofFIG. 1 including main memory 12, dual-loop DLL 30 and output circuit 22.Output circuit 22 includes an output latch 32 connected to an outputdriver 34. Output latch 32 is connected to main memory 12 via connectionline 35. Output driver 34 is connected to an output pad 36 whichprovides a data output signal DQ.

DLL 30 includes a forward path 38 having a first loop or coarse loop 40connected to a second loop or fine loop 42. In one embodiment, coarseloop 40 has a delay range up to 20 ns (nanosecond) to provide a widefrequency lock range. Fine loop 42 has a delay range from about 1 to 1.2ns to provide a tight locking. Coarse loop 40 receives an input clocksignal CLKref and a local clock signal CLK DLL on a feedback path 43.Fine loop 42 is responsive to coarse loop 40. Fine loop 42 also receivesthe CLKref signal and CLK DLL signal. Fine loop 42 outputs the localclock signal CLK DLL.

In a register-based all digital DLL, the phase jitter is primarilydetermined by the basic delay stage used in the delay line. Depending onthe variations of process, supply voltage and temperature (PVT), thedelay for one stage may vary from 130 ps to 350 ps. In a high-speedmemory system, this skew has to be further reduced to ensure propertiming and valid data windows. The dual loop embodiment illustrated inFIG. 2 can be used to reduce the skew. The fine loop 42 can be used toprovide fine delay interpolation and skew reduction after the coarseloop 40 is locked.

There are several ways to implement a fine delay line with a small delayresolution. FIGS. 3 and 4 illustrate two methods. FIG. 3 illustrates amethod involving eight taps with which the load is adjusted while FIG. 4illustrates a method involving a single tap with fast and slow paths.

The method in FIG. 3 employs a pair of series connected inverters 44 and45. The load can be adjusted through operation of switches 47-54 whichcan be used to switch capacitors 56-63 into the circuit. Animplementation for one of the capacitors, capacitor 63, is alsoillustrated. Each of the capacitors 56-63 may be implemented in asimilar manner. The capacitor 63 is implemented through a pair ofn-channel and p-channel transistors with their gate terminals connectedtogether and, in the case of the p-channel device, the remainingterminals connected to a voltage source (e.g. V_(DD)) and, in the caseof the n-channel device, the source and drain terminals are is connectedto ground. By adding or removing the capacitors 56-63, a delay can beachieved that can be increased or decreased in a step-wise fashion. Thatdelay is almost constant over PVT variations. The method of FIG. 3 has avery small, e.g. 0.3 ns intrinsic delay. Here, intrinsic delay refers tothe initial delay added to the loop when a fine loop is used. Theintrinsic delay will slow down the loop operation which is generally nota good feature.

The embodiment illustrated in FIG. 4 includes a slow path 65 which iscomprised of a first inverter 66, a second inverter 67, and amultiplexer 68. A fast path 70 is similarly comprised of a firstinverter 71, a second inverter 72, and a multiplexer 73. By varying thesize of the inverter in the slow path 65, a different delay resolutioncan be achieved. Thus, the embodiment of FIG. 4 utilizes different pathsto achieve a verniered delay. In contrast to the embodiment of FIG. 3,the delay varies with, or tracks, the variations in PVT, i.e. increasingin the slow corners and decreasing in the fast corners. However, a largeintrinsic delay is introduced because of the two inverters and themultiplexer for each delay tap (0.3 ns per tap).

An interleaved delay line constructed according to the present inventionis designed to use both delay interpolation methods to achieve:

-   -   (1) desired jitter performance under PVT variations;    -   (2) dynamic tracking of the delay variations without a large        number of delay taps; and    -   (3) quick and tight locking.

A block diagram of such an interleaved delay line 75 is shown in FIG. 5.A shift register 76 in combination with multiplexers 77 and 78 forms acontrol circuit that is used to select different delay taps with thedelay taps being selected alternately from the delay line comprised ofload adjusting taps and the delay line comprised of fast/slow-path taps.Initially, half of these delay taps are selected which gives an M-taptuning range for increasing or decreasing the delay. This arrangementgives more flexibility to eliminate the skew and other timing errorsunder PVT variations.

FIG. 6 illustrates a circuit for implementing the interleaved delay line75 of FIG. 5. In FIG. 6, a phase detector 80 receives the signalsCLKref, CLK DLL. The phase detector circuit 78 produces a FAST controlsignal and a SLOW control signal which are each comprised of pulses. Thenumber of pulses in the FAST and SLOW control signals is representativeof the difference in phase between the signals CLKref and CLK DLL. TheFAST control signal is used for advancing the phase of the signal CLKDLL while the SLOW control signal is used to retard the phase of thesignal CLK DLL. The FAST and SLOW control signals are input to a controlblock 82. The control block 82 outputs signals to control the capacitiveload of variable delay line 84 and to control the number of fast andslow paths connected in variable delay line 86. The variable delay line84 may be constructed as illustrated in FIG. 3 while the variable delayline 86 may be constructed as illustrated in FIG. 4. The signal OUT(which is the signal CLK DLL) is input via a feedback path, not shown,to the phase detector 80. A coarse locked loop is typically added infront of delay line 84, such that the delay line 84 is responsive to thecoarse locked loop and the signal CLK DLL is input to the coarse lockedloop. Through the implementation illustrated in FIG. 6, the advantagesof both the variable delay line 84 and variable delay line 86 can beobtained.

In an exemplary embodiment, eight delay taps (M=8) were used for eachdelay line and the typical delay of the load-adjusting tap for delayline 84 was approximately 30 ps (t_(dl)), although the delay varied from25 ps to 35 ps.

For the fast/slow variable delay line 86, a typical delay for each stagewas about 50 ps (t_(dp)) with a range of 35 ps-70 ps (per tap). Thetuning range of this interleaved delay line can be calculated as:$t_{tune} = {\frac{M}{2}\left( {t_{d1} + t_{dp}} \right)}$

For above given numbers, t_(tune) works out to be240 ps<t_(tune)<420which covers the coarse delay per stage over PVT variations. Theworst-case RMS jitter is below 35 ps and peak-to-peak jitter is lessthan 70 ps.

FIG. 7 illustrates another example of how the fine delay may be adjustedby adjusting the amount of drive. The phase detector 80 produces theFAST and SLOW control signals which are input to a selection controlblock 88. The selection control block 88 produces signals forcontrolling individual drive stages 90, 91, 92, 93. One of the drivestages, drive stage 91, is illustrated as a pair of parallel connectedinverters, and one of the inverters is illustrated in detail in FIG. 7A.Thus, the selection control block 88 determines if one or both pathswithin drive stages 90, 91, 92, 93 are used.

The following table compares the three types of delay discussed; namely,the load adjusting delay of FIG. 3, the drive adjusting delay of FIG. 7,and the fast/slow path adjustment of FIG. 4.

INTRIN- SIC DELAY DELAY INTER- DELAY T_(D) T_(D) T_(D) (TYP- POLATIONTAP (FAST) (TYPICAL) (SLOW) ICAL) Load ncap & 27 ps 34 ps 38 ps  300 psAdjusting (1) pcap Drive 2 inverters 20 ps 30 ps 45 ps  780 ps Adjusting(2) (in parallel) Fast/Slow 2 inverters 20 ps 50 ps 70 ps 1750 ps Path(3) each path (in serial) & 1 MUX

An interleaved fine delay line can use any two of these three methods toachieve fast and tight locks. It is possible that if the last twomethods are used, situations may arise in which the delay is variednonlinearly as shown in the simulation results of FIGS. 8A, 8B and 8C.Under those circumstances, duty cycle distortion of the output mayoccur. In terms of power distribution, the load adjusting delay is thebest whereas the fast/slow path adjustment is the worst.

FIGS. 8A, 8B and 8C are simulations based on using the load adjustingmethod of FIG. 3, the drive adjusting method of FIG. 7, and thefast/slow path method of FIG. 4, respectively.

While the present invention has been described in the context of a delaylocked loop, the present invention may also be utilized in a phase lockloop as illustrated in FIG. 9. In FIG. 9, a course loop is comprised ofa phase detector and control block 95 which controls a delay line 96.The fine loop is comprised of a phase detector and control block 98which controls an interleaved fine delay line 99 of the type, forexample, illustrated in FIG. 6. The output of the interleaved fine delayline 99 is input to the delay line 96 through a digitally controlledoscillator 100.

FIG. 10 illustrates a computer system 200 containing the SDRAM 10 ofFIG. 1 using the present invention. The computer system 200 includes aprocessor 202 for performing various computing functions, such asexecuting specific software to perform specific calculations or tasks.The processor 202 includes a processor bus 204 that normally includes anaddress bus, a control bus, and a data bus. In addition, the computersystem 200 includes one or more input devices 214, such as a keyboard ora mouse, coupled to the processor 202 to allow an operator to interfacewith the computer system 200. Typically, the computer system 200 alsoincludes one or more output devices 216 coupled to the processor 202,such output devices typically being a printer or a video terminal. Oneor more data storage devices 218 are also typically coupled to theprocessor 202 to allow the processor 202 to store data in or retrievedata from internal or external storage media (not shown). Examples oftypical storage devices 218 include hard and floppy disks, tapecassettes, and compact disk read-only memories (CD-ROMs). The processor202 is also typically coupled to cache memory 226, which is usuallystatic random access memory (“SRAM”) and to the SDRAM 110 through amemory controller 230. The memory controller 230 normally includes acontrol bus 236 and an address bus 238 that are coupled to the SDRAM110. A data bus 240 may be coupled to the processor bus 204 eitherdirectly (as shown), through the memory controller 230, or by some othermeans.

While the present invention has been described in connection withexemplary embodiments thereof, those of ordinary skill in the art willrecognize that many modifications and variations are possible. Suchmodifications and variations are intended to be within the scope of thepresent invention, which is limited only by the following claims.

1. A computer system, comprising: a processor having a processor bus; aninput device coupled to the processor through the processor bus; anoutput device coupled to the processor through the processor bus; amemory device coupled to the processor bus, the memory devicecomprising: a plurality of memory cells; circuits, clocked by a localclock signal, for writing information into and reading information outof said memory cells; and a dual locked loop for locking said localclock signal to an external reference signal, comprising: a first lockedloop for establishing a phase relationship between said local clocksignal and said reference signal; and a second locked loop responsive tosaid first locked loop and comprising: a delay line having a firstportion providing a variable amount of delay substantially independentlyof process, temperature and voltage variations and a second portion inseries with said first portion and providing a variable amount of delaythat substantially tracks changes in process, temperature, and voltagevariations; a control circuit for controlling the delay of said delayline; a phase detector for producing signals for input to said controlcircuit; and a feedback path for connecting an output of said delay lineto an input of said first locked loop and to said phase detector, saidlocal clock signal being available at said output of said delay line. 2.A computer system, comprising: a processor having a processor bus; aninput device coupled to the processor through the processor bus; anoutput device coupled to the processor through the processor bus; amemory device coupled to the processor bus, the memory devicecomprising: a plurality of memory cells; circuits, clocked by a localclock signal, for writing information into and reading information outof said memory cells; and a dual locked loop for locking said localclock signal to an external reference signal, comprising: a first lockedloop for establishing a phase relationship between said local clocksignal and said reference signal; and a second locked loop responsive tosaid first locked loop and comprising: a first circuit path having astepwise variable capacitive load; a second circuit path in series withthe first circuit path and having a plurality of stages each having atleast two paths; a control circuit for controlling the amount ofcapacitance in said first circuit path and the number of stages in saidsecond circuit path; a phase detector for producing signals for input tosaid control circuit; and a feedback path for connecting an output ofsaid delay line to an input of said first locked loop and to said phasedetector, said local clock signal being available at said output of saidsecond locked loop.
 3. A computer system, comprising: a processor havinga processor bus; an input device coupled to the processor through theprocessor bus; an output device coupled to the processor through theprocessor bus; a memory device coupled to the processor bus, the memorydevice comprising: a plurality of memory cells; circuits, clocked by alocal clock signal, for writing information into and reading informationout of said memory cells; and a dual locked loop for locking said localclock signal to an external reference signal, comprising: a first lockedloop for establishing a phase relationship between said local clocksignal and said reference signal; and a second locked loop responsive tosaid first locked loop and comprising: a first circuit path having astepwise variable capacitive load and a second circuit path in serieswith said first circuit path and having a plurality of stages eachhaving a variable amount of drive associated therewith; a controlcircuit for controlling the amount of capacitance in said first circuitpath and the number of stages in said second circuit path; a phasedetector for producing signals for input to said control circuit; and afeedback path for connecting an output of said second locked loop to aninput of said first locked loop and to said phase detector, said outputsignal being available at said output of said second locked loop.
 4. Acomputer system, comprising: a processor having a processor bus; aninput device coupled to the processor through the processor bus; anoutput device coupled to the processor through the processor bus; amemory device coupled to the processor bus, the memory devicecomprising: a plurality of memory cells; circuits, clocked by a localclock signal, for writing information into and reading information outof said memory cells; and a dual locked loop for locking said localclock signal to an external reference signal, comprising: a first lockedloop for establishing a phase relationship between said local clocksignal and said reference signal; and a second locked loop responsive tosaid first locked loop and comprising: a first circuit path having aplurality of stages each having a variable amount of drive associatedtherewith and a second circuit path in series with said first circuitpath and having a plurality of stages each having at least a fast and aslow path; a control circuit for controlling the number of stages insaid first circuit path and the number of fast paths and slow paths insaid second circuit path; a phase detector for producing signals forinput to said control circuit; and a feedback path for connecting anoutput of said second circuit to an input of said first circuit and tosaid phase detector.
 5. A computer system, comprising: a processorhaving a processor bus; an input device coupled to the processor throughthe processor bus; an output device coupled to the processor through theprocessor bus; a memory device coupled to the processor bus, the memorydevice comprising: a plurality of memory cells; circuits, clocked by alocal clock signal, for writing information into and reading informationout of said memory cells; and a dual locked loop for locking said localclock signal to an external reference signal, comprising: a first lockedloop for establishing a phase relationship between said local clocksignal and said reference signal; and a second locked loop responsive tosaid first locked loop and comprising: a delay line having a firstportion providing a variable amount of delay with little intrinsic delayand a second portion in series with said first portion and providing avariable amount of delay with a larger intrinsic delay; a controlcircuit for controlling the delay of said delay line; a phase detectorfor producing signals for input to said control circuit; and a feedbackpath for connecting an output of said delay line to an input of saidfirst locked loop and to said phase detector, said local clock signalbeing available at said output of said delay line.